Glasses doped with rare earths, optical devices produced from such glasses, and methods of production.
Optical fibers doped with a rare earth are used in producing lasers and amplifiers operative at various wavelengths. An optical signal can be amplified by ion fluorescence within the same operating wavelength region as the signal. Pump energy excites the rare earth metal causing it to fluoresce. This fluorescence, in a wavelength region in which an optical signal operates, amplifies the signal.
Different rare earth metals harmonize at different wavelengths. This makes it necessary to match a particular rare earth with a particular wavelength of interest. For example, excitation of erbium at 980 nm, or 1450 nm, provides pumped energy causing the erbium to fluoresce. Fluorescence in the 1520-1560 nm range allows a signal, operating in this wavelength region, to be amplified.
The significance of the 1550 nm wavelength in optical communication has led to extensive studies of erbium and its behavior as a rare earth dopant of glass. Other rare earth ions can be caused to fluoresce, and thereby amplify a signal, at their different characteristic wavelengths. Accordingly, while the present description is largely directed to erbium, it will be understood that the invention extends equally to other rare earth ions.
It is known that, in optical amplifiers, increased optical gain and decreased lifetime result from increasing the concentration of a fluorescing, rare earth ion. However, this is limited by clustering, a phenomenon that is occasioned by interactions between the same rare earth ions in close proximity to each other.
As the concentration of a rare earth ion, such as erbium, increases beyond a certain point in a glass, the fluorescence signal is quenched. This, in turn, decreases the lifetime of the important fluorescence transition, thereby decreasing the optical gain.
The dopant level of a rare earth ion, for example, erbium, in a glass is critical for controlling optical properties. The ions are raised from ground level to an excited state or level by energy pumped into the glass. The excited ions then undergo radiative decay while fluorescing. Optical signal amplification occurs by the stimulated emission of the signal photon in the excited state role of the ion.
The decay occurs in stages, referred to as fluorescing transitions, during which a proportion of the ion population decays to a given level. The initial level is at 36% of the maximum intensity, and is a reciprocal of the natural log xe2x80x9ce.xe2x80x9d The time required for this to occur is referred to as the lifetime. There are several levels of decay, and a lifetime for each may be measured, and reported, as e1, e2 and e3. However, the lifetime of the first level is usually considered the important level.
Another undesirable effect of clustering is known as upconversion. This occurs when one rare earth ion is said to xe2x80x9cstealxe2x80x9d a signal from another ion. This is referred to as xe2x80x9csharingxe2x80x9d fluorescence. The net result is that the thief goes to a higher energy level, but the victim goes to ground level. The desired population is thereby diminished.
It would, therefore, be highly desirable to provide a means of countering, or avoiding, this detrimental clustering of rare earth ions in glass. That would enable achieving the benefits obtainable with increased concentration of the rare earth in the glass.
It is a basic purpose of the invention to provide a solution to the problem of rare earth clustering in glass. Another purpose is to provide a method of increasing the concentration of a rare earth ion in a glass without quenching the fluorescence signal thereby obtainable. A further purpose is to provide an optical device with enhanced ability to amplify an optical signal and exhibit a longer fluorescence time. Still another purpose is to provide an improved amplifier for an optical signal. A still further purpose is to provide glasses having an enhanced ability to amplify an optical signal.
The article of the invention is a glass component in an optical system, the glass component comprising a silicate base glass doped with at least two oxides of Group III B elements, at least one of the elements being a rare earth.
The invention resides in part in a silicate glass containing at least two different rare earth/Group III B elements in its composition, at least one of the elements not having open shell xe2x80x9c4fxe2x80x9d orbitals.
The invention further resides in a method of decreasing clustering of a rare earth element in a silicate glass which comprises including in the glass composition at least one additional rare earth/Group III B element that does not have open shell xe2x80x9c4fxe2x80x9d orbitals.
The invention also resides in a method of producing a clad optical fiber which comprises forming the core of the fiber from a silicate glass which includes at least two rare earth/Group III B elements in it, composition, one of which does not have open shell xe2x80x9c4fxe2x80x9d orbitals.
Literature known to applicants, and deemed to have possible relevance, is provided in a separate document.
The present invention arose from studies directed at enhancing the amount of a rare earth element that could be included in a silicate glass while minimizing the tendency of such element to cluster in the glass. The problem of clustering is particularly severe in the production of clad optical fiber for devices such as optical amplifiers. However, it also finds application in planar devices, such as lasers and planar waveguides, as well as in MCVD, plasma, or axial vapor deposition processes.
The invention is founded on the discovery that the tendency of a rare earth to cluster can be counteracted by including a second rare earth in the glass composition. This expedient permits increasing the concentration of a desired rare earth metal in a glass while minimizing the loss of optical activity due to clustering of the rare earth in the glass.
The cause of the rare earth clustering is not known with certainty. However, a logical explanation, based on present evidence, is that rare earth ions tend to bond together, rather than dispersing in the glass. This could occur if there were a lack of bonding sites for the rare earth ions in the glass. It also suggests a glass structure such that the rare earth ions are incompatible with other glass ions due to differences in ionic size and charge. This is because the xe2x80x9c4fxe2x80x9d orbitals are so deeply buried that the electronic configuration is not likely to have any appreciable influence on mutual solubilities.
TABLE I sets forth the crystal ionic radii of several rare earths and transition metal ions of Group III B elements.
The crystal ionic radii of Si+4 and Ge+4, principal ions employed in optical fiber production, are 0.41 and 0.53 Angstrom units. As shown in Table I, the crystal ionic radii of rare earth ions are in the range of 0.8-1.1 Angstroms. This substantial difference in ionic radii strongly suggests why rare earth ions do not disperse well in simple glasses, such as silica and silica-germania glasses.
It is believed that the addition of a second Group IIII B ion provides compatible bonding sites for the initial ion. This reduces the tendency for the initial ion to cluster in a glass. By providing equal concentrations of the two Group IIII B ions, clustering could be reduced by as much as 50%.
The second ion must not have open shell xe2x80x9c4fxe2x80x9d orbitals. Further, if the glass is being used for a fluorescing effect, as in an amplifier, the second ion must have no interaction with the pump wavelength, or the fluorescence, of the initial ion. For example, by applying these criteria, candidates for the second ion in a glass system doped with erbium, or another rare earth, are yttrium (Y), lanthanum (La), lutetium (Lu), gadolinium (Gd), and europium (Eu). Y, La, Lu, and Sc have no open shell xe2x80x9c4fxe2x80x9d orbitals. This makes them ideal because they would not be expected to have absorptions that would interfere with any rare earth ion. Gd and Eu have no optical transitions that can interact with pump wavelengths of rare earth ions in the visible or infrared portions of the spectrum.
Fortunately, there is a strong likelihood of finding Group IIII B elements together in minerals. This likelihood is indicated by an X in a box in TABLE II.
The invention may be practiced by standard glass melting, or by known optical fiber-making procedures, depending on the products being produced. A clad product might be made using the well-known double crucible technique.
An optical fiber may be fabricated by the standard outside vapor deposition (OVD) process. The core cane is produced by delivering precursors for the chosen rare earths to an oxy-gas burner via a bubbler. The precursors may, for example, be rare earth organometallic materials. The principal glass former is delivered to the burner from a separate source in an amount properly proportioned to the amount of rare earth precursors. The precursor for the glass former may, for example, be a chloride, such as SiCl4, GeCl4 and/or AlCl3. Alternatively, it may take the form of organometallic materials, such as octamethyl cyclotetrasiloxane or Ge(OEth)4.
The mixture of products emanating from the oxy-gas burner is deposited on a rotating mandrel to produce a preform. The preform is consolidated into glass to form a blank that is stretched into a cane. The cane is then coated with a silica cladding layer. The clad blank is then consolidated and drawn into fiber.
This customary fiber-forming technique is prone to the occurrence of clustering, except at very low rare earth levels. Therefore, we prefer to employ a well-known glass melting method, double crucible melting, that is commonly used to produce clad products. In this method, two melting units, usually tubular and having a common center, are employed. A core, or interior, component glass is melted in a central melting unit. A cladding glass is melted in an exterior melting unit that surrounds the interior unit. The two glasses are drawn simultaneously to produce a clad product.
It has been found that the fluorescing properties of the rare earths can be substantially enhanced by incorporating them in a fluorinated, alkali silicate glass. In particular, the presence of fluorine in the glass enhances the fluorescence intensity and emissions that occur as an excited rare earth ion returns to its ground, or unexcited, state after being energized.
It is believed that the presence of fluorine in an alkali silicate glass alters the glass structure in a manner such that rare earth clustering is reduced. Optimum effects, in terms of an enhanced rare earth content with minimal clustering, are achieved by combining multiple rare earths with fluorine in an alkali silicate glass. The fluorescence signal is enhanced, thereby increasing optical gain, fluorescence emission (lifetime), and relative quantum efficiency.
Thus, a fluorinated, alkali silicate glass containing multiple rare earths is a unique glass system. At least about 4.5 wt. % F is required to be effective. Up to 13 wt. % may be added, but greater than about 7.5 wt. % is difficult to retain during melting.
A fluorinated alkali silicate base glass provides optimum lifetime values. Addition of up to about 2% Al23 serves to stabilize the glass. Other known additions, in particular, the alkaline earth metal oxides, may be present to the extent that they do not interfere with fluorescence or glass transmission. For example, CaO tends to create an opal, particularly in the presence of fluorine. The alkali metal oxides, Na2O and K2O, are interchangeable in so far as fluorescing properties are concerned. The choice is made based on desired physical properties and material cost. Rubidium and cesium oxides are also effective, but offer no advantage in optical properties.
Reduction in alkali content has the effect of broadening and flattening a fluorescence spectra curve. Potential substitutes for alkali metal oxides include oxides of lead, boron, phosphorous and aluminum. Replacement of alkali by lead is well known in lead silicate glasses. However, melting a fluorine-containing lead silicate glass becomes difficult because of the corrosiveness of lead fluoride. Hence, melting must be done in silica vessels, rather than platinum.
B2O3 may be substituted for alkali to some extent. However, complete substitution does not provide a batch that will melt to form a glass. Substitution of P2O5 for alkali tends to form crystals, such as Na3Gd(PO4)2.
It has been found that a substantial substitution of alumina for alkali can be made successfully. There is some tendency to phase separate at around 15% alumina, but clear glass is consistently attained at higher and lower alumina contents. Up to about 20 mole % Al2O3 may be employed, and at least 5% is preferred. Increased silica content enhances the amount of both alumina and fluorine that may be accommodated in a glass melt.